In WCDMA enhanced uplink the Node B sends scheduling commands to the user equipments (UE) at every Transmission Time Interval (TTI), which at present can be either 2 ms or 10 ms. Each user equipment maintains a serving grant (SG) by increasing or decreasing its transmitted bit rate as dictated by the scheduling grants received from the network. This change in the transmitted bit rate is in turn achieved by applying the selection of the Transport Format Combination (TFC) in the user equipment.
A scheduling grant or scheduling grant message provides an indication from e.g. the Node B of both serving and non-serving radio link sets to the user equipment about the maximum amount of uplink resources it may use. The scheduling grants are sent once per Transmission Time Interval or even less frequently. The serving grant (SG) represents the uplink resources that the user equipment is presently utilizing on the serving radio link set i.e. bit rate.
The following two types of grant channels have been specified [1] [2] in the standard according to prior art: Enhanced Relative Grant Channel (E-RGCH) and Enhanced Absolute Grant Channel (E-AGCH). The E-RGCH channel is transmitted by a serving radio link set (RLS) as well as by one or several non-serving RLS:s. This is shown in FIG. 1. The serving RLS transmits the commands “DOWN”, “UP”, and “HOLD”, which may correspond, e.g., to −1, +1, and 0 respectively. On the other hand, non-serving RLS:s transmits either “DOWN” or “HOLD”, which may correspond to +1 or 0 respectively. Only the serving RLS uses the E-AGCH channel to send the absolute grant, which corresponds to the maximum allowed bit rate for the user equipment.
In soft handover (SHO) the user equipment receives absolute and/or relative grants from the serving RLS, whereas the non-serving RLS sends only the relative grant. If any of the relative grants is “DOWN”, the user equipment estimates its New SG as follows according to prior art:New SG=last bit rate used−δ
The δ in the above equation may correspond to the user equipment transmit power or bit rate. If none of the relative grants is “DOWN” then the New SG will be according to the absolute/relative grants received from the serving RLS.
The existing solutions according to prior art imply at least two problems:
Firstly, not all scheduling grants are reliable. There is always a risk that the power of the grant is such that, e.g., a “DOWN” grant can be erroneously treated as a “HOLD” grant, which in turn results in too much interference in the system.
Secondly, the network is not aware whether the scheduling grants that it is transmitting are reliable or not.
Therefore, there is a need for solutions providing handling of unreliable scheduling grants at the user equipment and for reducing the occurrence of unreliable grants in the network.